覆盖元素Z索引

Question

我是新手。我的网站具有以下结构

<nav class="navbar fixed-top" id="mainNav">>

        <div class="container">
           //stuff goes here
        </div>
</nav>

<section id="signup" class="signup-section">
   <div class="container">
        //stuff goes here
      <form class="form-inline d-flex">
         //stuff goes here
      </form>
   </div>

</section>

然后我在导航栏上进行了以下z索引更改

 .container
    {
       position:relative;


      }

   .navbar
    {

     z-index:9;
     }


  .signup-section{

   z-index:1;

    }

   .form-inline{

    z-index:1;

    }

但是当我滚动dow时，导航栏仍位于signup-section / form-inline下方